ac3: update AC3PreferredStereoDownmixMode.

'11b' is reserved in the A/52 specification,
but newer encoders use it to indicate a Dolby
Pro Logic II compatible Lt/Rt downmix.
lavu: add AV_FRAME_DATA_DOWNMIX_INFO side data type.
ac3dec: set AV_FRAME_DATA_DOWNMIX_INFO side data.
NOT FOR COMMIT